Seismic performance assessment of multi-story RC buildings with soft-story collapse mechanism equipped with gapped inclined bracing (GIB)

Gapped inclined bracing (GIB) is an innovative retrofit system for buildings with soft-story collapse mechanism. The system consists of a brace with gap element that activates before the structure approaches post-capping negative stiffness and collapse, providing supplemental plastic deformation and enhancing ductility of the system. This technique is also a resilient solution as the damage is controlled at the retrofitted soft-story, reducing the cost and time of repair. The paper presents a numerical study to verify the application of GIB systems for modern RC buildings, where masonry infill panels may be removed from lower stories due to functional requirements for more open spaces. Based on nonlinear static analysis, a novel and efficient design method is proposed to determine the GIB design parameters. To verify the performance of the retrofitting methodology, 12 configurations of RC moment frames with 4 and 8 stories are designed according to modern seismic codes and subjected to nonlinear static and incremental dynamic analyses. The results are then used to conduct fragility analysis to assess classical performance levels including immediate occupancy, life-safety, and collapse prevention. The results show that using the proposed design of GIB elements, the ultimate drift capacity corresponding to the collapse of soft-structures are increased by a factor of two. Also, the probability of collapse is reduced by 50% increase in the median collapse hazard intensity. These highlight significant improvements in the safety and resilience of buildings with soft-stories.